--- - |2 Abstract Recent U.S. Supreme Court opinions have given rise to the question of whether persons suffering from a severe mental illness should be categorically exempt from the death penalty. This article presents a brief overview of relevant U.S. Supreme Court cases and the empirical evidence relevant to this question. We then present our findings on how actual capital jurors respond to and discuss engaging with evidence of mental illness, as drawn from in‐depth interviews collected as part of the Capital Jury Project. Existing research reveals that in the controlled situation of an experiment, evidence of mental illness is associated with votes for life rather than death. Similarly, actual capital jurors in our study reported anticipating that evidence of mental illness would make them less likely to vote for death. However, those jurors who dealt with mental illness in their case appeared to be less sensitive: they describe such evidence as having been overshadowed by the brutality of the crime; as indicative of the defendant's future dangerousness; as being confusing, especially as presented by experts; and as a manipulative attempt on the part of the defendant to trick the jurors. The findings suggest that capital jurors cannot reliably comprehend and account for evidence of mental illness and thus offer a compelling reason for the Court to exempt those suffering from a mental illness from the death penalty. - Behavioral Sciences & the Law, Volume 36, Issue 4, Page 470-489, July/August 2018.